Segmented agitator reactor

ABSTRACT

The present invention relates to an apparatus for carrying out chemical and physical processes, wherein flowable substances are mixed together, comprising a vertical, cylindrical vessel with inlets and outlets, as well as a central shaft and radial agitator blades arranged on the said central shaft and extending close to the wall. In this context, viewed in the peripheral direction, the agitator blades are curved and are combined in pairs of agitator elements one above the other to form groups, the blades of one element of the group being curved in a radially concave manner and the blades of the other element being curved in a radially convex manner.

The present invention relates to an apparatus for carrying out chemicaland/or physical processes, as well as to a process for the preparationof polyolefins.

Known reactors for the present purposes are invariably flow pipes oragitator vessels.

Ideal flow pipes offer the advantage that all transported particles aresubjected to the same residence time when passing there through whilethis is not the case in the agitator vessel; however, in the latter theparticles are mixed more effectively. If, therefore, a high degree ofmixing with the same residence time of all particles is to be attained,agitator vessel cascades are used. Such agitator vessel cascades can,however, only be realised in separate vessels, involving constructioncosts. Both reactor types, therefore, offer only limited possibilitiesin each case of influencing the fluid substance particles or substanceparticle mixtures in solid and/or liquid form treated therein.

It is, therefore, the object of the present invention to provide areactor for physical and/or chemical processes, combining theadvantageous characteristics of a flow pipe and an agitator vessel.

This object is attained according to the invention by an apparatus forcarrying out chemical and physical processes, wherein flowablesubstances are mixed together, comprising a preferably cylindrical(vertical) vessel with inlets and outlets, as well as a central shaftand radial agitator blades arranged on the said central shaft andextending close to the wall. Viewed in the peripheral direction, theagitator blades are curved and are combined in pairs of agitatorelements (one above the other) to form groups, the blades of one elementof the group being curved in a radially concave manner and the blades ofthe adjoining element being curved in a radially convex manner.

The specific characteristic of such a design resides in the following:The solid or liquid particles or gases passing through the tubularreactor, viewed in the direction of rotation, are displaced from thevertical central region outwardly towards the pipe wall by the concaveagitator blades, while the blades shaped in an opposite sense positionedabove or below the respective structural group convey these inwardlyagain. At the same time deflected flow lines come about on the bladeedges so that the particles viewed as a whole perform a flow path of aclosed helix. If a reactor of this type is not operated statically, buta vertical throughput is generated, e.g. by pumping or due to gravity,the further advantage, apart from very good mixing, of a distinctlyhomogeneous residence time distribution is brought about. Compared withconventional reactors, this, of course, also results in a substantiallyreduced reactor size.

A reactor according to the invention of this type may be used, inparticular, for producing homogeneous mixtures, for performing reactionsbetween solid and/or liquid and/or gaseous substances, for heating orcooling, for coating particles and similar uses. Its simple constructionalso permits, of course, processes performed under high pressure or athigh temperatures.

A further, very particular advantage is provided by the possibility toinfluence the process in a very well defined manner.

Since, as stated above, the residence time of the particles passingthrough the tubular reactor is very homogeneous, various measures mayalso be performed at various levels, such as, for example, locallyconfined cooling, feeding or withdrawing materials; cross flow scrubbingwith gases may, for example, be effected if solids particles are treatedin the reactor or a plurality of reaction steps with various reactioncomponents are carried out.

A further improvement, i.e., in particular, more speedy mixing andbetter defined residence time distribution may be attained by taperingthe blades towards their free ends, e.g. by a trapezoidal configurationof the blades, taking into account the increase of the peripheralvelocity in a radial direction.

Transverse feeding may also be improved by curving the blades in ahelicoidal manner, preferably in the form of a logarithmic orArchimedean spiral.

Moreover, the blades may also be pitched at an angle in relation to thelongitudinal axis of the shaft supporting the latter, in order to inducevertical forwarding. For example, the blades of the agitator elementsmay be pitched in opposite directions in such a manner that they movethe substance in their range of action towards one another, in order tointensify mixing.

A further possibility of influencing the components of the reactorcontents may be attained if between the individual agitator elements,i.e. between each of the agitator pairs, barrier disks are provided,guiding the substance during its passage outwardly towards the pipe wallor smoothing turbulence.

The preparation of polyolefins may be realised in a particularlyadvantageous manner by a reactor of this type.

In this case use is made of the effect that the action of the agitatorblade groups amounts to an improved agitator vessel, stagnant zones areabsent and a very high relative velocity of the reaction componentsmonomer, polymer and catalyst prevails, so that all products leaving thefirst group(s), have reacted to the same extent; in particular, thisalso involves a more homogeneous temperature distribution as comparedwith the state of the art.

Conventional processes for the preparation of polyolefins, in particularpolypropylene, are described in detail in the available literature.

Fluidised bed reactors, e.g.:

-   U.S. Pat. No. 4,003,712 by Union Carbide Corp.,-   EP 1080782 by Sumitomo Chem. Corp. LTD    Solvent processes, e.g.:-   WO 97/36942 by Dow chem. Corp.    Circulating multi-zone reactors, e.g.:-   WO 97/04015 by Monteil Technology CO BV (Basle)    Agitated gas phase reactors, e.g.:-   U.S. Pat. No. 4,921,919 by Standard Oil CO-   U.S. Pat. No. 3,639,377 by BASF AG

Mixing elements for reactors are described e.g. in DE 1,218265 or WO99/29406.

All processes stated have the object to withdraw reaction heat rapidlyand uniformly. In fluidised bed reactors this is brought about byrelatively large quantities of gas and a high relative velocity of theparticles in relation to one another. A disadvantage of this process, asalready mentioned above, is the high quantity of gas. For this purpose,normally a significant proportion of carrier gas is admixed to thereaction gas, which must subsequently again be recovered. Furthermore,the apparatus for generating the required high gas velocities arerelatively expensive and the reactors tend to form polymer deposits.

Solvent processes suffer from the disadvantage that the solvent usedmust be removed and recovered from the polymer. In all processesdescribed, the polymer is significantly back-mixed so that these may beconsidered as a continuously operated agitator vessel. Such back-mixedprocesses permit only a limited control of the polymer compositionduring the residence time of a polymer particle in the reaction zone.Furthermore, in such processes an appreciable proportion of the polymerparticles is already removed again from the reactor at an early stage ofthe reaction, resulting in a relatively low catalyst utilisation.

Another essential aspect is the quantity of transition material comingabout in a reactor when changing from product A to B. This quantity isdetermined essentially by the residence period performance of thereactor system.

FIG. 5 shows the residence period performance of a conventional CSTR.

As shown in FIG. 5, approximately 3 residence periods are required for aproduct transition in a CSTR to acquire a material property F. Thiscomes about in different manners in reactors, which, at least in part,are arranged in a cascade fashion such as e.g. loop reactors orhorizontal (vertical) reactors.

FIG. 6 shows the residence period performance of a loop or V-CSTRreactor.

As shown in FIG. 6, the product transition regarding material property Fmay theoretically be performed in loop or V-CSTR reactors by a cascadeof n=CSTR. It is disadvantageous that approximately 2 residence periodscontinue to product transition extending over the entire cascade. benecessary for a

The reactor according to the invention meets the following requirements:

-   -   High relative velocity of the polymer particles in relation to        one another,    -   High relative velocity of the polymer particles transversely to        the reactor axis    -   Velocity of the polymer particles in reactor axis proportional        to volume expansion of the polymer particles during the reaction        and minimal back-mixing.

According to the above mentioned output profile a system has beendeveloped which comprises the following characteristics: There isprovided a vertical, agitated reactor, subdivided into segments(groups). For conducting the reaction, it is, in principle, irrelevantwhether the main product flow is upwardly or downwardly directed. Thereactor comprises at least 2, preferably 4, particularly preferably morethan 12 theoretical segments, the following consideration applying toeach segment (see FIG. 7).

The design of the segment depends mainly on the gas velocities. Thepoint of operation of the reactor is in the range between the gasvelocity 0 and the fluidization limit, preferably in the range of0.5-0.8 times the gas velocity at the point of fluidization.

The residence period performance of such a reactor system may bedescribed by the graph (see FIG. 8) with regard to a product property F.

A reactor system of this type permits product transitions in less thanone residence period.

Each segment is equipped with an agitator element, permitting efficienthorizontal mixing and ensuring, as a result, a uniform distribution ofcooling agent over the entire reaction chamber as well as a highrelative velocity of the particles in relation to one another. Agitatorelements permitting such a mixing pattern, may, as stated above, be, forexample, in the form of a logarithmic spiral, the cross-sectionalprofile and design of the region at the agitator tips and the agitatoraxis being so configured that the mass flow remains substantiallyensured in the boundary zones, i.e. in the outermost and innermostoperating region of the agitator. It is particularly advantageous inthis context that agitators having an Archimedean or logarithmic spiralform are self-cleaning.

The height and diameter of the segments may be adapted to the prevailingkinetics of the catalyst system by appropriately selecting the reactorgeometry and the arrangement and configuration of the agitator elements.

In the region of the injection of the catalyst system a back-mixed zonemay be provided for better blending in of the catalyst and for a moreeffective heat dissipation. In this case the agitator element ismodified accordingly. For this purpose, for example agitator baffles orother internals are suited, which provide force mixing in the directionof the agitator axis.

For the withdrawal of the reaction heat liquid monomer is preferablyused as the cooling agent. If appropriate, inert components notparticipating in the reaction process may likewise be used. In thesimplest case the coolant is metered into the reactor from below andwithdrawn from above, regardless of the direction of the main flow ofthe product. It is also possible to provide individual segments withseparate dosing locations for a coolant and/or further reactioncomponents such as monomers, catalysts, activators, inhibitors etc. Inaddition, it is possible to influence the gas composition by withdrawingcooling agent or reaction components within the segments. The sameapplies to the use of barrier agents between segments for separating gascompositions within the reactor.

The present invention is elucidated in more detail by way of theaccompanying figures. There is shown in:

FIG. 1 the flow pattern compared to the state of the art,

FIG. 2 the basic structure of the reactor according to the invention and

FIG. 3 scrubbing of a polyolefin with inert gas

FIG. 4 agitator elements

FIG. 5 is a graphical representation of the residence period performanceof a conventional CSTR.

FIG. 6 is a graphical representation of the residence period performanceof a loop or V-CSTR reactor.

FIG. 7 is the description of the segment of the reactor system accordingto the invention.

FIG. 7 a is a listing of several formula symbols and definitions.

FIG. 8 is the residence period performance of a reactor system withregard to product property F.

On the left hand side, FIG. 1 shows the flow characteristics of a fluidmedium during its passage through an upright cylindrical vessel (1)comprising conventional central, vertical agitator elements, arranged ona driven coaxial shaft. The flow velocity decreases from the inside tothe outside, mainly due to boundary friction, so that the centralparticles exit the reactor at a clearly earlier stage than those in theboundary region, i.e. the residence time varies considerably.

The right hand side illustration shows the effect of the processaccording to the invention. The flow velocity of all particles isuniform, they pass through the reactor in a closed front as a plug flow,but with continuous mixing transversely to the reactor axis. Taking intoaccount the reaction velocity, the heat transition and the like, zonesmay thus be defined to which further reaction components, but alsoreaction inhibitors may be added from outside or via a hollow shaft; inthis manner it is possible, for example, to perform multi-stagereactions in a single reactor housing or also to vary different reactionparameters such as temperature, concentration in a reactor housing.

The conditions apply both to the preferred upright as well as to arecumbent or inclined arrangement of the reactor.

FIG. 2 illustrates the basic structure of the reactor according to theinvention.

It comprises an, in particular, upright, cylindrical vessel 1 includinga central shaft 2, which may be hollow. The shaft 2 supports agitatorblades 3, 4 serving as agitator elements 5, 6, each of them beingcombined to form groups 7 positioned one above the other and interactingwith one another, the blades 3, 4 being helically curved in oppositedirections, so that an agitator element 5 brings about feeding directedsubstantially radially to the exterior, while the other agitator element6 brings about feeding directed towards the interior. In this context,the spacing A between the agitator elements 5, 6 is advantageously lessthan the spacing B between the groups 7 from one another. The gap abetween the agitator blades 3, 4 and the vessel wall is adapted to thesize of the particles passing through the reactor, i.e. it may increasein the direction of flow with increasing particle sizes.

In order to increase the flow velocity at the reactor outlet, the lattermay be designed in a conical manner in the vicinity of the bottom 8.Substances may be introduced into the process or withdrawn from thelatter via a hollow shaft and corresponding outlet apertures (not shown)or via hollow agitator blades.

FIG. 3 shows a further application of the reactor according to theinvention.

The polyolefin particles obtained by the catalytic reaction in FIG. 3and grown to the catalyst contain non-reacted contents of gaseousmonomer.

The latter must be removed prior to further processing. For this purposethe product is passed (continuously) into a second reactor, equippedwith similar groups of agitator blades and scrubbed with inert gas underslight excess pressure or atmospheric pressure while being intenselyagitated, the residence period performance corresponding to flow pipeconditions, in turn bringing about absolutely uniform degassing.

The inert gas (N₂) may be introduced in counter-current, e.g. via thebottom, or, as indicated in the present case, via a hollow shaft and theagitator blades.

In the course thereof or thereafter it is possible, in principle, to addadditives to the polymer, e.g. colorants, which are in turn mixed veryhomogeneously with the polymer.

FIG. 4 shows various designs of the agitator elements in plan view, theblades 3, curved in one direction, being positioned in a plane differentfrom that of the blades 4 curved in the opposite direction.

It is preferred to use pairs of agitator blades, which may also bearranged cross-wise. Depending on the rotation velocity, the curvaturesof the blades may be designed in a more or less pronounced manner, anarrangement of the blades within a group, offset in relation to oneanother, in the adjoining planes being possible as well.

The following working example describes the preparation of polypropylenein a reactor, e.g. according to FIG. 2.

The raw materials used are the following:

-   Propylene: Preferably polymer grades having a purity of >99.8%-   Catalyst: 4^(th) generation Ziegler catalyst or other catalysts    suitable for the preparation of polypropylene-   Alkyl: preferably triethylaluminium-   Donor: Preferably a silane of the group dialkyldialkoxy silane of    the general formula RR′Si(OMe)2, R, R′ being equal or different, R,    R′=alkyl, isoalkyl, aryl, cycloalkyl . . .    Cyclohexyl-methyl-dimethoxysilane was used In the present example.-   Hydrogen: super pure.

In a continuously operated reactor (top down) having a length of 16.3 mand an initial diameter D2 of 1.74 as well as an end diameter of 0.7 mand an effective volume of 12 m³, filled with a polymer powder of mediumbulk density of 400 kg/m³, the catalyst is dosed continuously from aboveonto the agitator bed. The catalyst is suspended in propylene and isintroduced into the reactor together with pure propylene in a volume of1 t/h as a suspension having a concentration of 10%. The agitatingvelocity is 24 rotations/minute. Likewise from above triethylaluminium(alkyl) is metered in in a quantity of 250 g/t of the total propylenefeed dosage as well as cyclohexylmethyldimethoxy silane (donor) in aquantity of 21 g/t of the total propylene feed dosage, so that a molarratio of alkyl/donor of 20/1 is brought about. According to consumptionby polymerisation, further propylene is fed to the reactor from below ina pressure-regulated manner. This regulated monomer flow maintains thereactor pressure at 30 bar. The temperature regulation is effected suchthat per group (agitator pair), according to the temperature measured,liquid propylene for cooling is fed, e.g. laterally. The entirepropylene used for cooling is fed to a cooler via the reactor lid, whereit is condensed and fed again by a pump to the individual dosinglocations. Hydrogen in a quantity of 62 g/t of the propylene feed ismetered into the lowermost flow of the cooling gas. Hydrogen serves as aregulator for the molecular weight of the polymer. Propylene feed in aquantity of 5.1 t/h is likewise metered in at the bottom. The withdrawalof the polymer formed is effected continuously via a suitable withdrawalmeans such that the charge level of the reactor remains constant.Conventional methods for measuring the charge level of bulk materialsmay be employed for measuring the charge level. The average quantity ofwithdrawn polymer is 4.8 t/h, according to a mean residence time of thepolymer of 1 h. The polymer obtained has the following properties:

-   MFI (melt flow index): 12 g/10 min. (acc. to ASTM D 1238)-   XL (xylene-soluble content): 2.2% (acc. to ASTM D 5492)-   Average bulk density: 440 g/l (acc. to ASTM D 1895)

1. Apparatus for performing chemical and physical processes, whereinflowable substances are mixed together, comprising a cylindrical vesselwith inlets and outlets, a central shaft and radial agitator bladesarranged on the shaft, extending close to the wall, such that a gapbetween the blades and the wall is adapted to a size of particlespassing through the reactor, characterized by the following features: a)the agitator blades are designed in a curved manner when viewed in theperipheral direction and b) are combined in pairs of agitator elements,positioned one above the other, to form groups, c) the blades of oneelement of the group being curved in a radially concave manner and theblades of the other element being curved in a radially convex manner andd) the blades are tapered towards their free ends.
 2. Apparatusaccording to claim 1, wherein the curvature of the blades isspiral-shaped.
 3. Apparatus according to claim 1, wherein the curvatureis logarithmic or Archimedean.
 4. Apparatus according to claim 1,wherein the blades are pitched in the axial direction for verticalfeeding of substances in the axial direction.
 5. Apparatus according toclaim 4, wherein the blades of the agitator elements have pitchingangles in opposite directions.
 6. Apparatus according to claim 1,wherein barrier elements are provided between the groups of agitatorelements.
 7. Apparatus according to claim 1, wherein in the vessel wallspaced apart inlets and outlets are provided for feeding reactioncomponents and/or coolants to the flowable substances.
 8. Process forpreparing polymers in a tubular reactor, the reaction components beingmoved through the reactor in an axial direction and in the coursethereof being conveyed alternatingly towards and away from the reactoraxis by pairs of agitator blades, curved in opposite directions,combined to form groups one above the other, substantially normal to thereactor axis, thereby intensely mixing the reaction components, theblades being tapered towards their free ends.
 9. Process according toclaim 8, wherein an olefin polymer is placed into the reactor and amonomer and a catalyst system are added to the latter.
 10. Processaccording to claim 9, wherein the reactor content is cooled by adding aliquid monomer, the monomer evaporating in the course thereof and beingwithdrawn as a gaseous substance.
 11. Process according to claim 10,wherein the liquid monomer is fed to the reactor successively at aplurality of locations, viewed in the direction of conveyance of thepolymer.
 12. Process according to claim 9, wherein from the outside orvia a hollow shaft supporting the agitating blades or via the blades abarrier gas is introduced in sections into the reactor and radiallywithdrawn from the latter.
 13. Process according to claim 9, wherein theproduct exiting from the reactor is passed into a second reactor,wherein the product is freed of residual monomers by scrubbing with aninert gas, and wherein the second reactor comprises agitator bladeshaving at least two pair of agitator blades, curved in oppositedirections, combined to form groups one above the other, substantiallynormal to the reactor axis, with the blades being tapered toward theirfree ends.
 14. An apparatus according to claim 1, wherein thecylindrical vessel is a vertical vessel.
 15. A process according toclaim 13 wherein the inert gas is nitrogen.